Electrolytic etching and oxidizing of aluminum



July 17, 1956 TURNER 2,755,238

ELECTROLYTIC BTCHING AND OXIDIZING OF ALUMINUM Filed March 25, 1955 2 Sheets-Sheet 1 9 ELECTRO-ETCH OF HIGH PUP/TV AL.

HEAT TREATED E TCH RAT/O (R PRIOR ART ELE C TRO-E TCH 0F 5 HGH PUR/TY AL.

4 CHEM/CAL E TCH 0F COMMER/CAL AL.

I UNETCl-IED HIGH PUR/TY AL.

FORMAT/0N VOLTAGE (VOLTS) 1 Fl G. l f

INVENTOR.

TERRY E. TURNER T. E. TURNER July 17, 1956 ELECTROLYTIC ETCHING AND OXIDIZING OF ALUMINUM Filed March 25, 1955 2 Sheets-Sheet.

QQQQ QMMH OWL) Mg H/S ATT RNEYS 335 .6 a t n3 o .5: o l Qfi o t E5293 ELECTROLYTIC ETCHING AND 'OXI' D'IZING OF ALUMINUM Terry E. Turner, Aberdeen, Md., assignor to Spr'ague Electric Company, North Adams, Mass, a corporation of Massachusetts Application March 25, 1955, Serial No. 496,677

6 Claims.- (Cl. 204-33) R Oapacitance per unit area of etched foil 7 I e Oapacitance per unit area of plain (uneto hed) foil Where Re stands for the etch ratio, the etched and unetched foils are subsequently coated with oxide under the same conditions and the measured capacitances are those of the oxide coating.

A majority of electrolytic condenser electrodes manufactured today are formed of commercial purity aluminum, which has been chemically but non-electrolytically etched. As later will be shown, etch ratios of from 3 /2 to approximately 7, as the formation voltage of the oxide film is varied, are possible with this type of anode. Prior types of electrolytic etching have not found too wide an acceptance because they are not as simple as the chemical process. High purity aluminum cannot be satisfac torily etched by these chemical methods.

One of the objects of this invention is to provide a method whereby foil may be electrolytically etched so as to provide materially higher etch ratios than are obtained 'with prior art processes. A further object of this invention is to provide aluminum electrolytic capacitor anodes which have an extremely large effective surface area as compared to the known aluminum anodes. A still further object is to provide a method whereby high purity aluminum foil can be satisfactorily etched. These and 'other objects of the inventive concept will appear from the following description and claims, considered with the accompanying drawings wherein:

Fig. 1 is a graphical illustration of the improvements provided by the present invention; and

Fig. 2 is a schematic showing of how the process of the present invention can be carried out.

According to this invention these objects are obtained by heating aluminum sheets or foils to change their metallurgical characteristics, such changes including crystal orientation, grain size alteration, strain relieving and placing of the impurities present in it in solid solution prior to etching, as well as by anodically etching this sheet in an aqueous solution of a metal halide using a pulsating electrolytic current. It is not fully known why these enable higher than normal etch ratios to be obttained.

The metallurgical changes of the aluminum are carried 'out by heating in a furnace at from 500 to 660 C. prefterably 600 to 660 C. for a period of from 15 minutes to 24 hours. Any type of furnace, including an induc- States atent tion furnace may be used. A great deal of latitude is permissible within this step, depending upon the final metallurgical characteristics which are to be obtained.

After an aluminum foil has been heat treated as indicated, it is next electrolytically etched in an aqueous \metal halide solution. Any of the soluble halides are satisfactory for this step. The etching is preferably carried out during a period of from 2 to 20 minutes with an average current density from /2 to 5 amperes per square inch of anode surface at temperatures ranging from the freezing to the boiling points of the electrolyte used. The temperature of the electrolyte directly affects the size of the etched cubes obtained on the aluminum surface. In general, the lower the temperature of the electrolyte the larger the etched cubes obtained. Higher etching tem- :peratures around 100 C. appear to be the most satisfactory for direct current electrochemical etching when high formation voltages of 300 or more volts are to be used in electrolytically producing an oxide film on the etched aluminum electrode. The lower etching temperatures (50 to C.) are particularly suited for low purity aluminum and pulsating direct current electrochemical etching.

Aluminum foils that have been etched in accordance with the present invention may be anodically coated with oxide films according to the known methods presently used in the art at formation voltages of from 50 to 600 volts. In general, it is preferred to use as an oxidizing electrolyte an aqueous or non-aqueous solution of ammonia and boric acid, as shown for example in the United States Patent No. 2,444,725 issued to Burnham.

The results obtained with a two-hour heat treatment at 600 C. in accordance with the present invention are illustrated in Fig. l which also compares the invention with the prior art over the usual range of oxide forming voltages. The electrochemical etch for these curves was effected with pure direct current although comparable curves are obtainable with pulsating direct current. The term commercial purity aluminum as used in connection with this figure means aluminum which is 99.80%

to 99.88% pure; and the term high purity aluminurn means aluminum which is 99.99% pure. It is readily seen that electrolytically etched high and commercial purity aluminum foil which has been treated in accordance with this invention is more desirable for use in electrolytic condensers than is etched foil which has not been treated. Increased etch ratios of the magnitude'obtained with high purity foil are often obtained with aluminum of commercial purity. Etch ratios of at least 7.5 have thus become practical.

In a preferred embodiment of the invention aluminum foil is etched on an apparatus of the type indicated in Fig. 2. As will be seen from this figure, a sheet of heat treated aluminum foil is passed through an electro-etching tank between perforated cathodes next rinsed in tap Water; then rinsed in hot dilute nitric acid; and then again rinsed in tap water; and finally rinsed with deionized water before being passed through heated drying rolls and past infra-red lamps. A great many modifications of the apparatus pictured in Fig. 2 can be made as will be evident to those skilled in the art. The electrochemical etching operation following the heat treatment can be carried out preferably with a direct current although direct currents with power frequency alternating current ripples such as half-wave rectified current, and currents using square, saw-tooth, or other special wave forms may be employed with advantage within this invention. Sodium chloride is preferred as an electrolyte because of its low cost and outstanding performance, but soluble bromides, iodides, fluorides, and chlorides of any of the alkali or alkali earth metals may be satisfactorily employed at concentrations of from 0.1% to fully saturated. If desired, wetting agents can be added to the electrolytic etching bath. Precleaning of the aluminum is not necessary with this invention in order to obtain satisfactory results. The heat treated aluminum should not be exposed more than about one to two hours to intermediate temperatures, e. g. from about 100 C. to 500 C., before the etching.

Further examples of electrochemical etching within the scope of this invention are as follows with the aluminum foil heat treated by exposure to an atmosphere of about 600 C. for about two hours:

With the illustrated apparatus of Fig. 2 it is preferable to etch aluminum foil 99.99% pure in an aqueous electrolytic bath containing 13.5% NaCl held at 100 C. using an etching period of approximately 10 minutes and a current density of .8 of an ampere per square inch of foil. With pure direct current electrochemical etch an etch ratio of 8.0 will be realized. Under the same condi tions using direct current with 5% sixty cycle sine wave ripple imposed, slightly reduced etch ratios of about 7.1 are obtained.

A 99.99% pure aluminum foil that is not heat treated can be electrochemically etched in an aqueous electrolyte containing 1% of NaCl at a temperature of 51 C. and at an average current density of 0.8 ampere per square inch of foil with a direct current with 53% sixty cycle sine wave ripple imposed to obtain an etch ratio of about 6.3.

With commercially pure aluminum the use of pure direct current produces results that are somewhat sensitive to variations in the foil metallurgy so that diminished etch ratios occasionally result. In contrast the use of pulsating direct currents is much less susceptible to these variations and yields much more consistent etch ratios even without the heat treatment. Thus, it may be desired when etching aluminum foils or sheets of lowered purity to utilize the pulsating direct currents to power the electrochemical etch perhaps sacrificing slightly the etch ratio to achieve constant output. Aluminum foils as thin as 2 mils or less can be very successfully etched using this apparatus. The pulsations in the direct current should not cause the current to change in direction, that is, the anode should not become negative with re spect to the electrode inasmuch as this results in a marked loss of efiiciency. Pulsations having frequencies of from 30 to 1,000,000 cycles per second can be so used.

The etched and oxide-coated aluminum can be fashioned into electrolytic capacitors in any desired manner with a suitable electrolyte and opposing electrode. One effective technique is described in the above-mentioned Burnham patent.

The unoxidized, etched aluminum foil obtained by this invention presents a very pleasing appearance and can be advantageously used for decorative effects.

Those skilled in the art will realize that by virtue of the process set forth herein, electrolytic condensers of smaller size per unit of capacity may be readily formed.

It will also be realized that the invention herein involved is not to be limited specifically to electrolytic condenser electrodes, but may be employed wherever a large amount of oxide coating or active surface per unit of area is desired, so long as these applications come within the scope of the appended claims.

This application is a continuation-in-part of the copcnding application, Serial No. 238,405, filed July 25, 1951.

As many apparently widely different embodiments of this invention may be made without departing from the spirit and scope hereof, it is to be understood that the invention is not limited to the specific embodiments hereof except as defined in the appended claims.

What is claimed is:

1. The process of electro-etching aluminum foil so as to obtain a greater etTective surface area than the marginal dimensions of the surface which process comprises heating an aluminum foil having a purity of at least 99.88% at a temperature of from 500 C. to just below its melting point for from 15 minutes to 24 hours to orient the 200 planes of the metal crystals parallel to the foil surface, and after less than 2 hours exposure to temperatures between C. and 500 C., electrolytically etching said treated foil in an aqueous solution of a halide.

2. The process as defined in claim 1 in which the halide is sodium chloride.

3. The method of forming an electrolytic condenser electrode which comprises heating an aluminum foil of. about 99.99% purity to a temperature between 500 and 660 C. for from 15 minutes to 24 hours so as to orient the 200 planes of the crystalline structure of said foil parallel to its surface, after less than one hours exposure of the treated foil to temperatures between 100 C. and 500 C., electrolytically etching said surface of the foil in an aqueous solution of a halide, and anodically oxidizing said etched surface to aluminum oxide.

4. The process of claim 1 in which said electrolytic etching is effected by a direct current having alternating current ripples.

5. The process of preparing an etched aluminum foil, which process comprises the steps of heating an aluminum foil of at least 99.88% purity at a temperature of about 600 C. for from 15 minutes to 24 hours, and before the heat treated foil is exposed to temperatures between 100" C. and 500 C. for one hour, anodically etching it in an aqueous solution of a halide selected from alkali metal and alkaline earth halides.

6. The process of claim 5 in which the etching is effected with half-wave rectified alternating current.

References Cited in the file of this patent FOREIGN PATENTS 715,525 Great Britain Sept. 15, 1954 

1. THE PROCESS OF ELECTRO-ETCHING ALUMINUM FOIL SO AS TO OBTAIN A GREATER EFFECTIVE SURFACE AREA THAN THE MARGINAL DIMENSIONS OF THE SURFACE WHICH PROCESS COMPRISES HEATING AN ALUMINUM FOIL HAVING A PURITY OF AT LEAST 99.88% AT A TEMPERATURE OF FROM 500* C. TO JUST BELOW ITS MELTING POINT FOR FROM 15 MINUTES TO 24 HOURS TO ORIENT THE 200 PLANES OF THE METAL CRYSTALS PARALLEL TO THE FOIL SURFACE, AND AFTER LESS THAN 2 HOURS EXPOSURE TO TEMPERATURES BETWEEN 100* C. AND 500* C., ELECTROLYTICALLY ETCHING SAID TREATED FOIL IN AN AQUEOUS SOLUTION OF A HALIDE. 